#include <stdio.h>
#include <math.h>

double dist( double x1, double y1, double x2, double y2 );

int main()
{    
    double x1, y1, x2, y2;

    scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
    printf("dist = %.2f\n", dist(x1, y1, x2, y2));

    return 0;
}
double dist( double x1, double y1, double x2, double y2 )
{
    double a;
    if((x1-x2)==0&&(y1-y2)==0){
    a=0.00;
    }
    else {
       a=fabs((x2-x1)*(y2-y1)); 
    }
    return a;
}
